os: export ErrProcessDone variable in windows and plan9

[alwindoss]

Exposes ErrProcessDone variable in windows and plan9
also returns this error code instead of
errors.New("os: process already finished")

Fixes #42311

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Patch Set 2:

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Patch Set 3: Run-TryBot+1 Trust+1

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Patch Set 3:

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Patch Set 3:

Build is still in progress...
This change failed on linux-amd64-longtest:
See https://storage.googleapis.com/go-build-log/6e02c328/linux-amd64-longtest_3f55cdad.log

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Patch Set 3: TryBot-Result-1

2 of 22 SlowBots failed:
Failed on linux-amd64-longtest: https://storage.googleapis.com/go-build-log/6e02c328/linux-amd64-longtest_3f55cdad.log
Failed on windows-amd64-longtest: https://storage.googleapis.com/go-build-log/6e02c328/windows-amd64-longtest_b55ddef3.log

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Patch Set 3:

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Message from Bryan C. Mills:

Patch Set 4: Patch Set 3 was rebased

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Message from Bryan C. Mills:

Patch Set 4: Run-TryBot+1 Code-Review+2

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SlowBots beginning. Status page: https://farmer.golang.org/try?commit=2bd62b94

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Patch Set 4: TryBot-Result+1

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This PR is being closed because golang.org/cl/266997 has been merged.